In general, when a PSC (prestressed concrete) girder is manufactured, the lower portion of a girder is prestressed to endure load generated during a construction process, such as slabbing or packing. Tendons for prestressing the girder are arranged under the girder, and the section of the girder has very high clearance because excessive tension may generate tensile stress at the upper portion of a beam or compression stress of the lower portion thereof may exceed permissible compression stress. The PSC girder having high clearance has several disadvantages in that moment applied to the girder is increased due to increased self-weight as the PSC girder uses a great deal of concrete, in that aseismatic design of piers is not economical, and in that a bridge spanned above a road is deteriorated in economic efficiency since lots of banks must be made in front and in rear of an area, where a bridge is constructed, to secure a space under the bridge.
To solve the above problems, Korean Patent No. 30131, which was granted on Jun. 25, 2001, discloses a prestressed concrete girder capable of controlling tension force. The prestressed concrete girder, which includes an upper flange located on the lower portion of an upper board of a bridge for supporting the upper board to control load-carrying capacity of the bridge, a web part located on the lower portion of the upper flange for supporting the upper flange, and a lower flange located on the upper portion of a pier for supporting the web part, comprises: a tensed steel wire located in a longitudinal direction of the girder and tensed for supplementing the load-carrying capacity; at least one untensed steel wire located in the longitudinal direction of the girder; at least one connection member for fixing untensed steel wires induced from both ends of the girder; and a cut part formed at a predetermined area of the longitudinal direction to embody the connection member therein. Therefore, the prestressed concrete girder can control tension force of the bridge by tensing the untensed steel wire.
The prior art is not a method for constructing a bridge, but awarding to the prior art, the PSC girder bridge is constructed by a method of spanning a first-tensed PSC girder between piers, establishing surrounding spans during curing after concrete for slabs is poured on the girder in a construction field, and secondly tensing a compound section using an anchoring tool exposed to a side without any influence on the surrounding spans after the curing. However, in the prior art, the first-tensed PSC girder must endure load of the slabs poured in the construction field, and the compound section does not have an effect to remove excessive compression stress of the upper edge portion of the girder due to raised neutral axis even though prestress of the lower edge portion of the girder lost during the pouring of concrete for the slabs can be supplemented by the second tense. Therefore, a key point in design of the PSC girder bridge is to prevent the compression stress of the upper edge portion of the girder from exceeding permissible compression stress by traffic load. In addition, the prior art has a restriction in lowering clearance of the girder by increasing efficiency of tendons.
Meanwhile, according to demands of bridges with long span and to easily maintain spot portions, various methods for constructing a continuous bridge using a PSC girder have been developed. Not completely continuous bridges but continued bridges, which consider only trafficability and maintenance, were constructed before, but recently, construction methods of continuous bridges, which can continue all of the slabs and the girders and prevent cracks of connection parts, have been developed positively.
For this, Korean Patent Publication No. 2001-430, which was published on Jan. 5, 2001, discloses a method for constructing a continuous bridge using prestressed concrete girder having an exposed anchoring device. The method for constructing the continuous bridge using prestressed concrete girders, which includes simple steel wires of at least one group mounted on every girders, continuous steel wires of at least one group passing the plurality of girders, and/or connection steel wires of at least one group for connecting the girders, comprises the steps of: tensing the simple steel wires to the girders, spanning the girders between piers, connecting sheaths to connection parts of the girders and/or arranging the continuous and connection steel wires, pouring concrete for the connection parts and slabs, and applying tension force to the girders by tensing the continuous and/or connection steel wires; and re-tensing the continuous and connection steel wires to prevent droop or cracks of the continuous and connection steel wires and increase load-carrying capacity of the girders when active load acts to the girders and excessive droop and cracks occur due to aging of the girders during use.
The prior art has an advantage to reduce a construction period by simultaneously pouring concrete for the connection parts and concrete for the slabs in such a manner to span the plurality of the first-stressed girders between the piers, arrange the continuous and connection steel wires for the second tense, simultaneously pour and cure the concrete for the connection parts and concrete for the slabs, and then, apply the second tense.
However, the prior art has several problems in that it cannot release the excessive compression stress acting to the upper edge portions of the girders like the simple bridge construction method since the second tense is applied after the slabs are compounded with the girders, in that the first-tensed girders must impose the entire load of the slabs, and in that it cannot obtain a clearance reduction effect through continuity of the girders since the load of the slabs is applied not to the continuous girders but to the simple girders. Furthermore, cracks are generated on border surfaces between the connection parts and the PSC girders due to the first moment by positions of the tendons and due to the second moment by reaction force of continuous spots of the continuous bridge, which is a statically indeterminate structure. In fact, it has been reported that cracks are generated on bridges of national roads, which the prior art construction method was applied. FIG. 5 is a simple view showing the moment generating the cracks on the lower portions of the connection parts during the second tense according to the prior art construction method. In FIG. 5, (+) static moment is to generate tensile stress to the lower portion and compression stress to the upper portion.
To solve the problems of the simple bridges and the continuous bridges constructed by the prior arts, Korean Patent No. 25551, which was filed on Apr. 22, 2003, discloses ‘a method for constructing a simple bridge using PSC girders comprising the steps of: spanning PSC girders, which have the first tension force for enduring self-weight, between spot portions; applying the second tense while reapplying temporary load to the girders; removing the load while installing slabs’ and ‘a method for constructing a continuous bridge using PSC girders comprising the steps of: spanning a number of PSC girders, which have the first tension force for enduring self-weight, between spot portions; pouring concrete for connection parts between the PSC girders after continuously inserting the second tendons into sections of the neighboring PSC girders; applying temporary load while tensing the second tendons continuously inserted into the sections of the PSC girders; and removing the load while installing slabs’.
Such construction method has several advantages in that the second tension force is applied only to the girders, where the slabs are not compounded, because the second tense is performed while a controllable loading device previously applies load, which is applied while the slabs are installed, in that the construction method can prevent tensile cracks generated on border surfaces between the connection parts and the PSC girders due to the second tense and reloading performed at the same time when the continuous bridge is constructed, and in that the moment occurring the girders is reduced and a bridge of low clearance or long span can be constructed since the continuous girders endure the load of the slabs. However, the construction method has a disadvantage in that there is some loss in construction efficiency and economical efficiency since a device for reloading and removing temporary load is required.